A machine failure is often characterized by a sudden increase in the vibration acceleration level. We propose a wireless and battery-free vibration sensor and a method for condition monitoring and predictive maintenance of vibrating objects such as machines and engines. The principal part of our system is a piezoelectric transducer that acts both as a vibration intensity sensor and in addition as an energy source for the sensor circuitry. Our system consists of a battery-free sensor attached to the monitored object that transmits vibration data to a remote Base Station (BS) wirelessly via Bluetooth Low Energy (BLE) beacons. The vibration data is interpreted remotely by the BS via time-domain readout by analyzing the advertising time Tadv of the beacons received by the BS, that can monitor the vibrations and can send a standby signal to the machines in case of emergent faults.